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Cortical columns
A cortical column is a group of neurons in the brain cortex which can be successively penetrated by a probe inserted perpendicularly to the cortical surface, and which have nearly identical receptive fields. The human cortical column is composed of 6 layers. Each layer receives and sends signals to different parts of the brain. The human cerebral cortex is a roughly 2 mm thick sheet of neuronal cell bodies that forms the external surface of the telencephalon. The columnar functional organization, as originally framed by Vernon Mountcastle, states that neurons that are horizontally more than a half mm from each other do not have overlapping sensory receptive fields. An important distinction is that this rule is functional in origin, and reflects the local connectivity of the cerebral cortex. Connections "up" and "down" within the thickness of the cortex are dramatically denser than connections that spread from side to side. Mammalian cerebral cortex The mammalian cerebral cortex, the grey matter encapsulating the white matter, is composed of layers. The human cortex is roughly 2.4 mm thick. The number of layers is the same in all mammals, but varies throughout the cortex. In the neocortex 6 layers can be recognized although many regions lack one or more layers, fewer layers are present in the archipallium and the paleopallium. The dolphin cortical column is composed of only 5 layers. Although the dolphin brain is larger than the human brain, the dolphin has a need to sleep and still surface to breathe. It accomplishes this by having one-half of its brain sleep at a time. The reptilian cortex has only three layers. Columnar functional organization The columnar functional organization, as originally framed by Vernon Mountcastle, suggests that neurons that are horizontally more than 0.5 mm (500 µm) from each other do not have overlapping sensory receptive fields, and other experiments give similar results: 200–800 µm (Buxhoeveden 2002, Hubel 1977, Leise 1990, etc.). Various estimates suggest there are 50 to 100 cortical minicolumns in a hypercolumn, each comprising around 80 neurons. An important distinction is that the columnar organization is functional by definition, and reflects the local connectivity of the cerebral cortex. Connections "up" and "down" within the thickness of the cortex are much denser than connections that spread from side to side. Size of cortex From the size of the cortex and the typical size of a column, it can be estimated that there are about two million function columns in humans. There may be more if the columns can overlap, as suggested by Tsunoda et al.. Hubel and Wiesel studies Hubel and Wiesel followed up on Mountcastle's discoveries in the somatic sensory cortex with their own studies in vision. A part of the discoveries that resulted in them winning the 1981 Nobel Prizehttp://nobelprize.org/medicine/laureates/1981/ was that there were cortical columns in vision as well, and that the neighboring columns were also related in function in terms of the orientation of lines that evoked the maximal discharge. Hubel and Wiesel followed up on their own studies with work demonstrating the impact of environmental changes on cortical organization, and the sum total of these works resulted in their Nobel Prize. See Also *Ocular dominance column *Cortical minicolumn External links and references *Mission to build a simulated brain begins "the initial phase of Blue Brain will model the electrical structure of neocortical columns - neural circuits that are repeated throughout the brain. These are the network units of the brain, says Markram. Measuring just 0.5 millimetres by 2 mm, these units contain between 10 and 70,000 neurons, depending upon the species. Once this is complete, the behaviour of columns can be mapped and modelled" * Blue Brain Project Aims to simulate a cortical column * Category:Neural networks